1. Field of the Invention
The present invention relates to an imaging lens mounted to imaging apparatus using an imaging element of CCD (Charge Coupled Device), CMOS (Complementary Metal Oxide Semiconductor) or the like, for example, a digital still camera, a cellular phone with a camera, and an information portable terminal (PDA: Personal Digital Assistance) and the like.
2. Description of Related Art
According to an imaging element of CCD, CMOS or the like, in recent years, small-sized formation and high pixel formation have been considerably progressed. Therefore, also a main body of the imaging apparatus as well as a lens mounted thereto which are small-sized, having a high function are requested. In order to achieve small-sized formation, shortened formation of a total length and small diameter formation (small-sized formation in a diameter direction orthogonal to an optical axis) are needed. Further, generally, according to an imaging optical system, other than small-sized formation, a telecentric performance, that is, making an angle of incidence of a principal ray to an imaging element near to parallel (an angle of incidence at the imaging surface becomes near to zero relative to a normal line of the imaging surface) is requested. In order to ensure the telecentric performance, it is advantageous to arrange an optical diaphragm aperture as proximate to an object side as possible. JP-A-2005-292235 discloses an imaging lens having a constitution having 3 lenses as a total thereof, and arranging an optical diaphragm aperture to the object side the most. Further, JP-A-2004-302058 discloses an imaging lens having a constitution having 3 lenses as a total thereof and arranging an optical diaphragm aperture between a first lens and a second lens.
Meanwhile, as high pixel formation of an imaging element has been progressed, it is requested for an imaging apparatus for taking an image of a still picture to provide a mechanical shutter to achieve a reduction in signal-noise at the imaging element. When the shutter is provided, it is advantageous to arrange the shutter proximately to an optical diaphragm aperture in order to reduce shading. On the other hand, according to the imaging lens having 3 lens constitution, in order to ensure the telecentric performance as described above, it is advantageous to arrange an optical diaphragm aperture as proximate to an object side as possible, for example, before or after the first lens. However, when a shutter mechanism is arranged before the first lens, the most object side, an arrangement becomes disadvantageous in view of small-sized formation. Hence, it is conceivable to arrange the shutter mechanism at inside of the lens system, between the first lens and the second lens. For that purpose, in the field of an imaging lens having 3 lens constitution it is desired to develop a lens having a high aberration performance in correspondence with high pixel formation while sufficiently ensuring an air interval between the first lens and the second lens for arranging the shutter mechanism. Although according to Embodiment 3 of JP-A-2005-292235, the interval between the first lens and the second lens is ensured to be comparatively wide to construct a lens constitution advantageous for arranging the shutter mechanism, it is desired to develop a lens having a high aberration function which is further advantageous for arranging the shutter mechanism. Further, although it is advantageous to use a lens having a special shape of an aspherical surface or the like in order to realize high function formation by a small number of lenses, in this case, it is preferable to select a pertinent lens material in consideration of a production performance and cost.